"I think Outlook leaves a lot of room for a fast competitor. There's a lot of user dissatisfaction out there, and it will be interesting to see what the market looks like once there's a strong open-source alternative." - Mozilla Contributor.
Sunbird, which was donated to Mozilla by corporate volunteer OEone, will now receive more development time and attention given that Firefox and Thunderbird 1.0 are complete. The intention will be to integrate Sunbird into Thunderbird so that users can do things like search across e-mail documents and calendar entries, and click a button to turn an e-mail message into a calendar task or reminder.
Despite its similarity to Microsoft Outlook, Microsoft claim to be unconcerned by development of "Lightening":
"Thunderbird does not offer an equivalent comparison to Microsoft Office Outlook. Customers expect much more than simple calendaring and the ability to send and receive e-mails. The integration of Exchange and Outlook far outweighs any feature that Thunderbird may deliver, and we don't see it as being applicable for serious business use." - Microsoft.